Bacillus thuringiensis (B.t.) is a soil-borne bacterium that produces pesticidal crystal proteins known as delta endotoxins or Cry proteins. Cry proteins are oral intoxicants that function by acting on midgut cells of susceptible insects. An extensive list of delta endotoxins is maintained and regularly updated at http://www.lifesci.sussex.ac.uk/home/Neil_Crickmore/Bt/intro.html.
European corn borer (ECB), Ostrinia nubilalis (Hübner), is the most damaging insect pest of corn throughout the United States and Canada, and causes an estimated $1 billion revenue loss each year due to crop yield loss and expenditures for insect management (Witkowski et al., 2002). Transgenic corn expressing genes encoding Cry proteins, most notably Cry1Ab, Cry1Ac, or Cry1F, provide commercial levels of efficacy against ECB.
Despite the success of ECB-resistant transgenic corn, the possibility of the development of resistant insect populations threatens the long-term durability of Cry proteins in ECB control and creates the need to discover and develop new Cry proteins to control ECB and other pests. Insect resistance to B.t. Cry proteins can develop through several mechanisms (Heckel et al., 2007, Pigott and Ellar, 2007). Multiple receptor protein classes for Cry proteins have been identified within insects, and multiple examples exist within each receptor class. Resistance to a particular Cry protein may develop, for example, by means of a mutation within the toxin-binding portion of a cadherin domain of a receptor protein. A further means of resistance may be mediated through a protoxin-processing protease. Thus, resistance to Cry toxins in species of Lepidoptera has a complex genetic basis, with at least four distinct, major resistance genes. Lepidopteran insects resistant to Cry proteins have developed in the field within the species Plutella xylostella (Tabashnik, 1994), Trichoplusia ni (Janmaat and Myers 2003, 2005), and Helicoverpa zeae (Tabashnik et al., 2008). Development of new high potency Cry proteins would provide additional tools for management of ECB and other insect pests. Cry proteins with different modes of action produced in combination in transgenic corn would prevent the development ECB insect resistance and protect the long term utility of B.t. technology for insect pest control.